A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The catalysts contain rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium also comprises methyl acetate, water, methyl iodide and the catalyst. Conventional commercial processes for carbonylation of methanol include those described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and 5,144,068, the entire contents and disclosures of which are hereby incorporated by reference. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105, the entire content and disclosure of which is hereby incorporated by reference. The reaction solution is withdrawn from the reactor and purified to obtain acetic acid.
During the purification of the reaction solution, several process streams are recycled to the reactor. The recycling of these streams often causes various impurities to build up in the carbonylation reactor. Corrosion metal contaminants, such as compounds containing iron, nickel, molybdenum, chromium, and the like are some examples of these impurities. These corrosion metal contaminants are formed due to the corrosive nature of the reaction solution and/or process streams. The corrosion metal contaminants build up in the carbonylation process streams as the process is operated over extended periods of time. As the corrosion metal contaminants are recycled to the reactor, these contaminants may build up in sufficient quantities, which can interfere with the carbonylation reaction and/or accelerate competing reactions such as the water-gas shift reaction or methane formation. The presence of these corrosion metal contaminants can have adverse effects on the process, in particular, a consequent loss in yield based on carbon monoxide. Further, corrosion metal contaminants can react with halogen catalyst promoters, which reduces the stability of the catalyst system. The need therefore exists for reducing the amount of corrosion metal contaminant in a carbonylation reaction system.
Several processes have been used to remove corrosion metal contaminants. U.S. Pat. No. 4,007,130 describes contacting a catalyst solution containing metallic corrosion products with an ion exchange resin in its hydrogen form to recover the catalyst solution free of the metal contaminants. U.S. Pat. No. 4,628,041 describes recovering rhodium and iodine values in the manufacture of acetic acid by carbonylation by precipitating the rhodium to separate it from corrosion metal contaminants. U.S. Pat. No. 4,894,477 describes a process that uses strongly acidic ion exchange resins in the lithium form to remove corrosion metal contaminants. U.S. Pat. No. 5,466,876 describes a chelating resin that is selective for the removal of corrosive metals rather than to carbonylation catalyst and co-promoter. U.S. Pat. No. 5,731,252 describes contacting the catalyst solution with an ion exchange resin bed, in the lithium form, and using a sufficient amount of water to decrease the concentration of alkali metal ions to optimize removal of corrosion metal contaminants.
In addition to corrosion metal contaminants, other impurities may be present and/or may tend to build up in the carbonylation process, thus causing adverse effects such as process equipment fouling. These impurities may be difficult to remove with exchange resin beds when corrosion metal contaminants are present since the corrosion metal contaminants tend to bind to exchange resin beds more aggressively than do other impurities.
As will be appreciated by one of skill in the art, there is an incentive to improve existing processes for the production of acetic acid by reducing the amount of impurities.